Fuel cell power plants are well-known and are commonly used to produce electrical energy from reducing and oxidizing fluids to power electrical apparatus such as apparatus on-board space vehicles. In such power plants, a plurality of planar fuel cells are typically arranged in a stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids. Each individual cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reactant or reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. In a cell utilizing a proton exchange membrane (“PEM”) as the electrolyte, the hydrogen electrochemically reacts at a surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
In operation of PEM fuel cells, the membrane is saturated with water, and the anode electrode adjacent the membrane must remain wet. Product water formed at the cathode electrode is removed from the cell by evaporation or entrainment into a gaseous stream of either the process oxidant or reducing fluid. In fuel cells containing porous reactant flow fields and adjacent porous cooler or water transport plates, as described in U.S. Pat. No. 6,331,366 owned by the assignee of all rights in the present invention, a portion of the water maybe alternatively removed as a liquid through the porous reactant flow field and water transport plate to a circulating cooling fluid.
While having important advantages, PEM cells are also known to have significant limitations especially related to liquid water transport to, through and away from the PEM, and related to simultaneous transport of gaseous reducing fluids and process oxidant fluids to and from the electrodes adjacent opposed surfaces of the PEM. The prior art includes many efforts to minimize the effect of those limitations. Use of such fuel cells to power a transportation vehicle gives rise to additional problems associated with water management, such as preventing the product water from freezing, and rapidly melting any frozen water during start up whenever the fuel-cell powered vehicle is operated in sub-freezing conditions.
Known fuel cells typically utilize a coolant system supplying a flow of cooling fluid through the fuel cell to maintain the cell within an optimal temperature range. Where the cooling fluid is a solution including water it also must be kept from freezing. It is known to utilize an antifreeze solution such as ethylene glycol and water or propylene glycol and water as a cooling fluid in such coolant systems. However, such antifreeze solutions are known to be adsorbed by and poison the catalysts that form electrodes. Furthermore, those antifreeze solutions that have low surface tensions result in the solutions wetting any wetproofed support layers adjacent to the cell catalysts, thereby impeding diffusion of reactant fluids to the catalysts, which further decreases performance of the electrodes. Also, the vapor pressure of typical antifreezes is too high, resulting in excessive loss rates of the antifreeze solutions through fuel cell exhaust streams or from steam produced in boilers of fuel processing components of fuel cell power plants. Therefore coolant systems of fuel cells that utilize an antifreeze solution are known to be sealed from the electrodes, so that the solution is not in direct fluid communication with the electrodes. Sealing the coolant system from direct fluid communication with the cell and hence with the product water formed at the cathode electrode results in decreased cell performance due to less efficient removal of the product water. Fuel cells with sealed coolant plates typically remove product water as an entrained liquid. This requires a tortuous serpentine flow path with a resultant high pressure drop. An example of such a cell is shown in U.S. Pat. No. 5,773,160. That type of cell is not suitable for operating at near ambient reactant pressure that is a preferred operating pressure for many fuel cell systems. The decreased performance of cells with sealed coolant plates requires that additional cells be used to satisfy the design power requirement. The additional cells combined with heavier coolers associated with sealed coolers results in an increase in weight and volume of a fuel cell power plant, which is undesirable for a fuel cell used to power a vehicle.
Recently, it has been disclosed to utilize a “direct antifreeze solution” that passes through a porous water transport plate so that the direct antifreeze solution is in direct, uninterrupted fluid communication with a cathode catalyst of the fuel cell. Such direct antifreeze solutions are described in U.S. Pat. No. 6,316,135 to Breault et al. that issued on Nov. 13, 2001, U.S. Pat. No. 6,361,891 to Breault et al. that issued on Mar. 26, 2002, and U.S. Pat. No. 6,365,291 to Margiott that issued on Apr. 2, 2002, all of which Patents are owned by the assignee of all rights in the present invention. The direct antifreeze solutions described in those three Patents exhibit thermophysical properties such that they do not excessively poison the cathode or anode catalysts during normal operation of fuel cells utilizing those direct antifreeze solutions.
Exemplary direct antifreeze solutions disclosed in those Patents include “alkanetriol direct antifreeze solutions”, such as water and an alkanetriol selected from the group consisting of glycerol, butanetriol, and pentanetriol. Another direct antifreeze solution is characterized in those patents as follows: a “special direct antifreeze solution” having; 1. a freezing point of at least −30 degrees Centigrade (hereafter “° C.”); 2. a surface tension greater than 60 dynes per centimeter (hereafter “dyne/cm”) at a cell operating temperature of about 65° C.; 3. a partial pressure of antifreeze above the solution at about 65° C. that is less than 0.005 mm of mercury (hereafter “mm Hg”); and, 4. a capacity of being oxidized by catalysts of the fuel cell at fuel cell voltages.
The inventors of the invention described herein undertook extensive experimentation with the direct antifreeze solutions described above, and while performance was enhanced over known antifreeze solution cooling fluids within cells having porous cooler plates, nonetheless performance decay has been observed. In particular, the alkanetriol glycerol was utilized as the direct antifreeze solution within an operating fuel cell, and a performance decay on the order of one-half (0.5) millivolts per hour at about 500 milliamps per square centimeter (hereafter “mASC”). The performance decay was recovered by operating the fuel cell with water as the cooling fluid. The exact decay mechanism is not yet clearly understood. One theory of the performance decay is that it is due to dehydration of the proton exchange membrane or of an ionomer within the catalyst on the membrane. The dehydration may come about because the partial pressure of water above the glycerol direct antifreeze solution is about 80% of the partial pressure of pure water as a cooling fluid. Another theory is that the performance decay is due to absorption of the glycerol direct antifreeze solution onto either the anode or cathode catalysts, thus poisoning the catalysts and reducing their effectiveness.
Accordingly there is a need for a fuel cell that may be operated in sub-freezing conditions by use of a direct antifreeze solution cooling fluid that does not produce any performance decay of the fuel cell, that also minimizes free water within the system that may be frozen when the fuel cell is not operated, and that does not require isolating an antifreeze cooling fluid from the cathode and anode electrodes within a sealed coolant system.